The present invention relates to a multi-output switching power source circuit to conduct constant-voltage control using a magnetic amplifier or a magamp.
FIG. 1 shows a conventional circuit configuration of a multi-output switching power supply circuit to conduct constant-voltage control using a magnetic amplifier or a transducer.
The power supply 20 includes a transformer 20 including a primary side which includes a direct-current (dc) power source 1, an input smoothing condenser or capacitor 2, a starting resistor 3, a pulse width modulation controller 4, detecting resistors 5 and 6, a capacitor 7, a smoothing choke coil 8, a rectifying diode 9, a commutating diode 10, and a main switch 11, e.g., an n-type metal-oxide semiconductor transistor (to be simply referred to as an NMOS hereinbelow).
On the primary side of the transformer 20, a primary winding 21 and an auxiliary winding 22 are disposed. The transformer 20 includes a secondary side including secondary windings 23a, 23b, 23c, etc. for respective output sections A, B, C, etc., respectively.
The output section A includes a magnetic amplifier 31, a rectifier diode 32, a commutator diode 33, a smoothing choke coil 34, a capacitor 35, a dummy resistor 36, a constant-voltage control circuit 37, detecting resistors 38 and 39, a transistor 40, a resistor 41, and a diode 42. The other output sections B, C, and the like are configured substantially in the same manner as for the output section A. Each section includes a load RL in its output section.
Referring next to FIGS. 2 and 3, description will be given of a principle of operation of the magnetic amplifier shown in FIG. 1.
As can be seen from a graph of FIG. 2, the magnetic amplifier 31 is on when a pulse current having a pulse width of x xcexcs (micro sec.) is flowing in the circuit. Even when the pulse current repeatedly changes its state between an on state and an off state, the magnetic amplifier 31 is in a magnetized state which conducts reciprocation between point A corresponding to a maximum value of the pulse current and point B corresponding to a state in which the current or a magnetic field associated therewith is zero as shown in FIG. 2. The magnetic amplifier 31 is kept retained in the on state. However, when a current slightly flows through the amplifier 31 in a direction opposite to that of the pulse current, that is, when a reset current flows therethrough, the state of magnetization of the amplifier 31 changes to a state corresponding to point C. The amplifier 31 therefore turns off. In this situation, even when voltage E is applied to the amplifier 31 in a forward direction, the current does not flows at once. According to a relationship
Magnetic flux (xcfx86)=Product of Voltage and Time (Txc3x97E),
the current starts flowing with a delay of time, i.e., rising time of
xcex94T=xcex94xcfx86/E.
By controlling the rising time delay xcex94T by the reset current, the pulse width modulation is carried out. In this case, if
x=xcex94T,
no current flows at all. In other words, by regulating the width of xcex94xcfx86 of the amplifier 31, the pulse modulation is conducted in a range of pulse current from 0% to 100%.
Subsequently, description will be given of operation of a multi-output switching power source circuit of the prior art shown in FIG. 1.
In the power supply circuit, the dc power source section 1 generates a dc input voltage V1. The input smoothing capacitor 2 smoothes the voltage V1.
The PWM (power width modulation) control circuit 4 produces a control signal V4 having a predetermined frequency and a pulse width corresponding to detected voltage, which is detected as below. The auxiliary winding 22 on the primary side of the transformer 20 generates an alternating-current (ac) voltage. The rectifying diode 9 rectifies the ac voltage into a pulsating voltage. The smoothing choke coil 8 and the smoothing capacitor 7 smooth the pulsating voltage to obtain an output dc voltage. The resistors 5 and 6 divides the dc voltage. The PWM control circuit 4 detects a change in the divided voltage to thereby produce the detected voltage. The secondary winding 23a produces an ac voltage determined by a turn ratio, i.e., a ratio between a number of turns of the primary winding 21 and that of the secondary winding 23a. By producing an ac voltage proportional to the ac voltage in the secondary winding 23a by the auxiliary winding 22, the PWM control circuit 4 controls the pulse width according to the change in the ac voltage to resultantly keep the output voltage at a fixed value. The NMOS 11 turns on or off the input dc voltage V1 according to the control signal V4 to generate an ac voltage V11 having a predetermined frequency and a pulse width associated with the detected voltage. The transformer 20 transforms the ac voltage V11 to produce ac voltages V23a, V23b, V23c, etc. respectively from the secondary windings 23a, 23b, 23c, etc. according to turn ratios respectively between the primary and secondary windings.
The magnetic amplifier 31 converts the ac voltage V23a through on/off control using a reset current into an ac voltage V31 having a pulse width associated with the reset current. The rectifier diode 32 rectifies the ac voltage V31 to produce a pulsating voltage V32. The voltage V32 has electromagnetic energy of, which is accumulated in the smoothing choke coil 34. When the diode 32 on the rectifying side is off and the diode 33 on the commutating side is on, the electromagnetic energy is supplied to the smoothing capacitor 35. The capacitor 35 smoothes the pulsating voltage V32 into an output do voltage. The output section A feeds the do voltage V to the load RL.
The magnetic amplifier 31 stabilizes the dc voltage using a hysteresis characteristic. That is, the resistors 38 and 39 detects variation in the dc output voltage. The constant-voltage control circuit 37 adjusts the reset current 142 for the magnetic amplifier 31 to stabilize the dc voltage. During a period in which the amplifier 31 is off, the adjusted reset current 142 is delivered via the transistor 40, the resistor 41, and the diode 42 to the amplifier 31. This resultantly controls the rising edge of a period in which the amplifier 31 is on to thereby stabilize the de output voltage.
Referring next to FIG. 4, description will be given of a circuit configuration of a second example of the multi-output switching power supply circuit of the prior art using a magnetic amplifier to control a constant voltage.
The multi-output switching power source circuit includes a main output section A and a plurality of subsidiary output sections B, C, etc. Among the output sections, the main output section A has a maximum output and small load variation. A switching duty ratio on the primary side is controlled by a negative feedback operation according to variation in an output voltage from the main section A. Each of the subsidiary output sections produces an output voltage. For the output voltage, the magnetic amplifier controls and produces an ac voltage having a duty ratio determined according to the output voltage from the main output section A.
The multi-output switching power supply circuit of the conventional example 2 shown in FIG. 4 includes, on the primary side of a voltage transformer 60, a de power source 51, an input smoothing capacitor 52, a starting resistor 53, a PWM control circuit 54, a capacitor 55, a smoothing choke coil 56, a rectifying diode 57, a commutating diode 58, and an NMOS 59.
The transformer 60 includes a primary winding 61 and a subordinate winding 62 on the primary side and secondary windings 63, 64, 65, etc. on its secondary side.
The main output section A includes a rectifying diode 71, a commutating diode 72, a smoothing choke coil 73, a smoothing capacitor 74, a dummy resistor 75, and a constant-voltage control circuit 76. The main output section A is connected to a load RL1. The subordinate section B includes a magnetic amplifier 79, a rectifying diode 80, a commutating diode 81, a smoothing choke coil 82, a smoothing capacitor 83, a constant-voltage control circuit 84, resistors 85 and 86, a transistor 87, a resistor 88, and a diode 89. The subordinate output section B is connected to a load RL2. The subordinate output section C is configured substantially in the same way as for the subordinate output section C and is connected to a load RL3.
The secondary winding 63 of the transformer 60 produces an ac voltage. The rectifying diode 71 rectifies the ac voltage into a pulsating voltage V71 having electro-magnetic energy. The smoothing choke coil 73 accumulates the electro-magnetic energy. When the rectifying diode 71 is off and the commutating diode 72 is on, the electro-magnetic energy is fed to the smoothing capacitor 74. The capacitor 74 smoothes the pulsating voltage V71 into a dc output voltage V1 to be applied to the dummy resistor 75 and the load RL1. When the output voltage V1 changes, the constant-voltage control circuit 76 detects the voltage change to produce a detection signal V76. The signal V76 is supplied to the PWM controller 54, which conducts negative feedback control for a pulse width of an ac voltage V59.
According to a duty ratio determined by the PWM controller 54, the secondary winding 64 of the transformer 60 generates an ac voltage V64 corresponding to a turn ratio between the primary winding 61 and the secondary winding 64. The ac voltage V64 is fed via a magnetic amplifier 79 of the subordinate output section B to be rectified by a diode 80 into a pulsating voltage V80. The voltage V80 has electro-magnetic energy, which is accumulated in the smoothing choke coil 82. When the rectifying diode 80 is off and the commutating diode 81 is on, the electro-magnetic energy is supplied to the smoothing capacitor 83. The smoothing capacitor 83 smoothes the pulsating voltage V80 into a de output voltage V2. The subordinate output section B feeds the dc output voltage V2 to the load RL2. The resistors 85 and 86 detects variation in the voltage V2, and the constant-voltage controller 84 accordingly adjusts a reset current 189 for the magnetic amplifier 79 to stabilize the dc output voltage V2. When the NMOS 59 is off, that is, when the rectifying diode 80 is off, the reset current is delivered via the transistor 87, the resistor 88, and the diode 89 to the magnetic amplifier 79. As a result, the rising time of the on period of the magnetic amplifier 79 is controlled to stabilize the dc output voltage V2. The subordinate output section C operates in almost the same way as the subordinate output section B.
The Japanese Patent No. 2927734 describes a low-loss output circuit, which is conventional example 3 associated with the technical field of the present invention. As shown in FIG. 5, the prior art is a low-loss output circuit including a magnetic amplifier MA connected to a secondary winding N2 of a voltage transformer producing an ac voltage having a rectangular waveform, a rectifying element Q1 including an MOS field-effect transistor (FET) on a rectifying side, a rectifying element Q2 including an MOS-FET on a flywheel side, and a smoothing choke coil CH and a smoothing capacitor C which smooth outputs from the rectifying elements to produce a dc output voltage. The smoothing choke coil CH supplies a signal to drive the rectifier element Q2.
In the configuration, the smoothing choke coil CH to smooth the output from the rectifier element including an MOS-FET delivers a driving signal to the rectifying element Q2 on the flywheel side to turn the MOS-FET Q2 on. Therefore, the driving signal is fed to the MOS-FET on the smoothing choke coil side to turn the MOS-FET on during a period from when polarity of the secondary winding of the transformer is changed to when the magnetic amplifier is saturated after a lapse of its predetermined controlled period of time. According to the Japanese Patent Ser. No. 2927734, this resultantly reduces power loss on the flywheel side and hence efficiently lowers the overall loss.
In the multi-output switching power supply circuit in which constant voltage control is conducted using a magnetic amplifier as above, a diode is generally employed in its rectifying circuit. The use of such a diode in the rectifier circuit leads to a problem that power loss due to a voltage drop in the diode lowers conversion or transformation efficiency.
There also arises a problem as below. Since circuits of large-scale integration are operated with a lower voltage as a power source voltage thereof, there are highly required output voltages of +3.3 V, +2.5 V, +1.8V, etc. However, the voltage drop of the diode is almost fixed, about, 0.4 V. This consequently results in a problem. That is, when the output voltage of the power supply circuit is reduced, the power loss in the rectifier circuit including such a diode becomes relatively larger in the overall loss in the power source circuit. This further lowers the conversion loss and hinders the lowering of the output voltage.
The problem of the multi-output switching power source of the prior art will now be described by referring to the configuration of the conventional example 2 shown in FIG. 4.
Assume that the power source circuit of the prior art does not include the dummy resistor 75. In this situation, when the load RL1 connected to the main output section A is reduced, for example, as in a no-load state and hence a load current thereof becomes equal to or less than a critical current of the smoothing choke coil 73, energy accumulated in the choke coil 73 is stored in the smoothing capacitor 74 to resultantly increase the dc output voltage V1. To suppress the increase in the output voltage V1, a control operation is conducted to reduce a time width of the on state of the main switch (NMOS) 59. In this situation, the pulse width of the ac voltage generated by the secondary winding 63 becomes smaller depending on cases. Therefore, it is impossible to guarantee the period of time or the voltage-time product necessary for the magnetic amplifier 79 in the subordinate output section B (between the voltage applied across the magnetic amplifier 79 and the time required for the saturation of the magnetic amplifier 79). To cope with the difficulty, a dummy resistor 75 is arranged in the main output section A. The resistor 75 keeps the time width of the on state of the main switch (NMOS) 59 to thereby guarantee the voltage-time product necessary for the magnetic amplifier 79. This leads to a problem that the dummy resistor continuously requires power and hence the power efficiency is lowered. This leads to an additional problem. That is, for the dummy resistor 75, a radiator is required to cool the dummy resistor 75 or an electronic dummy circuit is required, and hence the number of constituent components is increased.
Moreover, the low-loss output circuit of the conventional example 3 achieving constant voltage control by a magnetic amplifier and including a synchronous rectifying element using an MOS-FET has an object in which the smoothing choke coil supplies a driving signal to the rectifier element on the flywheel side to prevent a state in which the MOS-FETs Q1 and Q2 are on at the same time to thereby suppress occurrence of a short-circuit current. The invention is therefore not associated with output voltage control in the technical field of the present invention.
In addition, no consideration has been given to influence of failure on the primary side of the voltage transformer upon the secondary side thereof in the conventional example 3. For example, in the circuit configuration of FIG. 6, the voltage transformer includes a core which is reset by a free resonance caused by inductance of the transformer and drain-source capacitance of an NMOS 91 including a gate electrode. As a result, the gate electrode of the NMOS 93 is applied with a flyback voltage as shown in FIG. 7, and hence its conductive state is deteriorated.
It is therefore an object of the present invention, which has been devised to solve the problems, to provide a multi-output switching power supply circuit which can increase power source conversion efficiency to easily increase the number of outputs.
Another object of the present invention is to provide a multi-output switching power supply circuit including a main output section and subordinate output sections in which the subordinate output section can produce dc output voltages in a stable state without employing a dummy resistor and an electronic dummy resistor in the main output section.
In accordance with a first aspect of the present invention, there is provided a multi-output switching power supply circuit, comprising a dc power source for generating a dc input voltage; a detecting circuit for detecting a voltage value of a second ac voltage generated by a first subordinate winding, the first subordinate winding constituting a voltage transformer including a primary side, a primary winding, a core, a secondary side, and a secondary winding; a switching circuit for turning on or off the dc input voltage according to a control signal generated by detecting variation in the voltage value of the second ac voltage and thereby producing a first ac voltage having a predetermined frequency and a pulse width corresponding to the second ac voltage; a control circuit for generating the control signal according to variation in the voltage value of the second ac voltage detected by said detecting circuit; an active clamp circuit for passing an exciting current through the primary winding of said voltage transformer during an off period of said switching circuit and for thereby resetting the core of said voltage transformer, said dc power source, said detecting circuit, said switching circuit, said control circuit, and said active clamp circuit being arranged on the primary side of said voltage transformer; and a plurality of output sections disposed on the secondary side of said voltage transformer, each of said output sections comprising a magnetic amplifier for controlling, according to a reset current, on or off of a third ac voltage generated on the secondary winding through voltage conversion of the first ac voltage by said voltage transformer and for thereby generating a fourth ac voltage having a pulse width corresponding to the reset current; a rectifying circuit for rectifying the fourth ac voltage into a pulsating voltage; a smoothing circuit for smoothing the pulsating voltage into a do output voltage and for applying the dc output voltage to a load; and a voltage control circuit for detecting variation in the dc output voltage and for generating the reset current to conduct negative feedback control for the fourth ac voltage. Moreover, said rectifying circuit comprises a first NMOS transistor which is turned on or off according to a voltage value of a fifth ac voltage generated on a second subordinate winding disposed on the secondary side of said voltage transformer and which thereby generates the pulsating voltage; said smoothing circuit comprises a smoothing capacitor for smoothing the pulsating voltage into the dc output voltage and for applying the dc output voltage to a load; a choke coil for accumulating electromagnetic energy associated with the pulsating voltage; and a second NMOS transistor which turns on, when said first NMOS transistor is off, according to a voltage value of a sixth ac voltage generated on a third subordinate winding disposed on the secondary side of said voltage transformer and which thereby supplies the electromagnetic energy from the choke coil to the smoothing capacitor; and said magnetic amplifier is arranged between the secondary winding and said first NMOS transistor.
In accordance with a second aspect of the present invention, there is provided a multi-output switching power supply circuit, comprising a dc power source for generating a dc input voltage; a detecting circuit for detecting a voltage value of a second ac voltage generated by a first subordinate winding, the first subordinate winding constituting a voltage transformer including a primary side, a primary winding, a core, a secondary side, and a secondary winding; a switching circuit for turning on or off the dc input voltage according to a control signal generated by detecting variation in the voltage value of the second ac voltage and thereby producing a first ac voltage having a predetermined frequency and a pulse width corresponding to the second ac voltage; a control circuit for generating the control signal according to variation in the voltage value of the second ac voltage detected by said detecting circuit and a level of a detection signal detected by a voltage variation detecting circuit; an active clamp circuit for passing an exciting current through the primary winding of said voltage transformer during an off period of said switching circuit and for thereby resetting the core of said voltage transformer, said dc power source, said detecting circuit, said switching circuit, said control circuit, and said active clamp circuit being arranged on the primary side of said voltage transformer; a main output section disposed on the secondary side of said voltage transformer, comprising a first rectifying circuit for rectifying a seventh ac voltage generated on a first secondary winding through voltage conversion of the first ac voltage by said voltage transformer and for thereby generating a first pulsating voltage; a first smoothing circuit for smoothing the first pulsating voltage into a first dc output voltage and for applying the first dc output voltage to a load; and the voltage variation detecting circuit for detecting variation in the first de output voltage into a detection signal and for supplying the detection signal to said control circuit; and a plurality of output sections disposed on the secondary side of said voltage transformer, each of said output sections comprising a magnetic amplifier for controlling, according to a reset current, on or off of an eighth ac voltage generated on the secondary winding through voltage conversion of the first ac voltage by said voltage transformer and for thereby generating a ninth ac voltage having a pulse width corresponding to the reset current; a second rectifying circuit for rectifying the ninth ac voltage into a second pulsating voltage; a second smoothing circuit for smoothing the second pulsating voltage into a second dc output voltage and for applying the second dc output voltage to a load; and a voltage control circuit for detecting variation in the second dc output voltage and for generating the reset current to conduct negative feedback control for the ninth ac voltage. Said first rectifying circuit comprises a first NMOS transistor which turns the seventh ac voltage on or off at timing synchronized with switching timing of said switching circuit and which thereby generates the first pulsating voltage; said smoothing circuit comprises a first smoothing capacitor for smoothing the first pulsating voltage into the first dc output voltage and for applying the first dc output voltage to a load; a first choke coil for accumulating electromagnetic energy associated with the first pulsating voltage; and a second NMOS transistor which turns on when said first NMOS transistor is off, and which thereby supplies the electro-magnetic energy from the choke coil to the smoothing capacitor; said second rectifying circuit comprises a third NMOS transistor which is turned on or off according to a voltage value of a tenth ac voltage generated on the second subordinate winding disposed on the secondary side of said voltage transformer; said second smoothing circuit comprises a second smoothing capacitor for smoothing the second pulsating voltage into the second dc output voltage and for applying the second dc output voltage to a load; a second choke coil for accumulating electromagnetic energy associated with the second pulsating voltage; and a fourth NMOS transistor which turns on, when said fourth NMOS transistor is off, according to a voltage value of an 11th ac voltage generated on a third subordinate winding disposed on the secondary side of said voltage transformer and which thereby supplies the electromagnetic energy from the second choke coil to the second smoothing capacitor; and said magnetic amplifier is arranged between the secondary winding and said third NMOS transistor.
In accordance with a third aspect of the present invention, there is provided a multi-output switching power supply circuit, comprising a dc power source for generating a dc input voltage; a detecting circuit for detecting a voltage value of a second ac voltage generated by a first subordinate winding, the first subordinate winding constituting a voltage transformer including a primary side, a primary winding, a core, a secondary side, and a secondary winding; a switching circuit for turning on or off the dc input voltage according to a control signal generated by detecting variation in the voltage value of the second ac voltage and thereby producing a first ac voltage having a predetermined frequency and a pulse width corresponding to the second ac voltage; a control circuit for generating the control signal according to variation in the voltage value of the second ac voltage detected by said detecting circuit and a level of a detection signal detected by a voltage variation detecting circuit; an active clamp circuit for passing an exciting current through the primary winding of said voltage transformer during an off period of said switching circuit and for thereby resetting the core of said voltage transformer, said dc power source, said detecting circuit, said switching circuit, said control circuit, and said active clamp circuit being arranged on the primary side of said voltage transformer; and an output section disposed on the secondary side of said voltage transformer, comprising a magnetic amplifier for controlling, according to a reset current, on or off of a third ac voltage generated on the secondary winding through voltage conversion of the first ac voltage by said voltage transformer and for thereby generating a fourth ac voltage having a pulse width corresponding to the reset current; a rectifying circuit for rectifying the fourth ac voltage into a pulsating voltage; a smoothing circuit for smoothing the pulsating voltage into a dc output voltage and for applying the dc output voltage to a load; and a voltage control circuit for detecting variation in the dc output voltage and for generating the reset current to conduct negative feedback control for the fourth ac voltage. Moreover, said rectifying circuit comprises a first NMOS transistor which turns the third ac voltage on or off at timing synchronized with switching timing of said switching circuit and which thereby generates the pulsating voltage; said smoothing circuit comprises a smoothing capacitor for smoothing the pulsating voltage into the dc output voltage and for applying the dc output voltage to a load; a choke coil for accumulating electromagnetic energy associated with the pulsating voltage; and a second NMOS transistor which turns on when said first NMOS transistor is off, and which thereby supplies the electro magnetic energy from the choke coil to the smoothing capacitor; said first NMOS transistor includes a gate electrode, a source electrode, and a drain electrode, said gate electrode being connected to a winding end side of the secondary winding, said source electrode being linked with a ground side, said drain electrode being coupled with a winding start side of the secondary winding; said second NMOS transistor includes a gate electrode, a source electrode, and a drain electrode, said gate electrode being connected to the winding start side of the secondary winding, said source electrode being linked with a ground side, said drain electrode being coupled with an output port of said magnetic amplifier; said magnetic amplifier is arranged between the gate electrode of said first NMOS transistor and the drain electrode of said second NMOS transistor; and the reset current is supplied to a winding start side of said reset winding and is outputted to the ground side.
In accordance with a fourth aspect of the present invention, the multi-output switching power supply circuit of one of the first to third aspects described above further comprises a diode having a small voltage drop in a stage after said second NMOS transistor in parallel with said second NMOS transistor.
In accordance with a fifth aspect of the present invention, in the multi-output switching power supply circuit of one of the first to third aspects described above, said active clamp circuit comprises a capacitor connected to a winding end side of the primary winding of said voltage transformer; and an NMOS transistor including a gate electrode, a source electrode, and a drain electrode, said gate electrode being connected to a signal delivered from said control circuit, said signal being opposite in phase to the control signal generated from said control circuit; said source electrode being coupled with an output from said switching circuit, said drain electrode linked with said capacitor, the signal opposite in phase to the control signal generated from said control circuit having deadtime preventing an event in which said switching circuit and said NMOS are on at the same time.
In accordance with a sixth aspect of the present invention, in the multi-output switching power supply circuit of one of the first to third aspects described above, said third ac voltage has a pulse width necessary for saturation of said magnetic amplifier.
In accordance with a seventh aspect of the present invention, in the multi-output switching power supply circuit of the second aspect described above, said eighth ac voltage has a pulse width necessary for saturation of said magnetic amplifier.